Confusion about `Random`

[Paul Backus]

On Thursday, 22 December 2022 at 16:23:16 UTC, jwatson-CO-edu 
wrote:
> I am confused about why Program 1 produces random output but 
> Program 2 does not.

The code you have posted as "Program 2" is incomplete, and cannot 
be compiled as-is. I have made some changes in order to get it to 
compile and produce useful output, resulting in the following 
program:

#### sparrow_core.d

```d
module sparrow_core;

import std.random;

Mt19937 rnd; // Randomness

void init_random(){
     // Seed the RNG with the clock
     rnd = Random( unpredictableSeed );
}

double rand01(){
     // Uniform random sampling in [0,1)
     return uniform( 0.0, 1.0, rnd);
}

void init_SPARROW(){
     // Populate necessary global structures
     init_random(); // --- RNG
}
```

#### app.d

```d
module app;

import sparrow_core;
import std.stdio;

void main(){
     init_SPARROW();
     foreach (i; 0 .. 6)
         writeln(rand01());
}
```

When I compile and run the above program, I get the following 
output:

```
0.289729
0.39377
0.693163
0.232496
0.388511
0.840994
```

So, as far as I can tell, there is nothing wrong with your code, 
and the random number generator is working as intended.

Most likely you have made a mistake somewhere in the part of the 
code that you did not post, and that mistake is what's causing 
the lack of randomness you observed in the output.